1. Field of the Invention
The present invention relates to an improved method and apparatus for making electrical interconnects between power semiconductor devices and a substrate module and specifically to interconnecting power semiconductor devices with a substrate module using a formed conductive foil element or a formed flexible interconnection assembly comprised of a supporting dielectric film and conductive foil elements.
2. Description of the Related Art
Power semiconductor devices, or power integrated circuits (ICs), used as switches or rectifiers in power electronic circuits, and the like, often operate at considerably higher power and current density levels than other semiconductor devices.
Generally power ICs are enclosed or packaged in a housing formed with exposed conductive surfaces serving as IC input/output terminals or gates. Typically the packaged IC is positioned on a substrate module such as a lead frame or on a printed circuit board (PCB), and conductive surfaces of the IC are electrically interconnected with the substrate module or PCB. Generally, the substrate module or PCB provides an electrical interface between the IC and a larger electrical system.
The most common process for making electrical interconnections between ICs and a substrate module is by wire bonding. Automated wire bonding machines attach or mechanically bond a first end of a conductive wire to a conductive surface on the IC and attach or mechanically bond a second end of the wire to a corresponding conductive pad on the substrate module. The mechanical bonding occurs when the automated wire bonding machine contacts an end of a wire to a conductive pad and applies a pressure force in combination with applying thermal energy, ultrasonic energy or both. The applied pressure force and energy deforms the wire end and generates a mechanical bond between the wire end and the conductive pad. The mechanical bond may comprise a diffusion bond, chemical bond, adhesive bond or the like, depending upon the bonding process and equipment. Usually, the shape and surface area of the bond are controllable by the parameters of the automated wire bonding machine. After bonding a first end of the wire to the IC, a second end of the wire is mechanically bonded to an appropriate conductive pad on the substrate module forming a wire loop between the bonded ends. Generally, the size, shape and length of the wire loop are controllable by the parameters of the automated wire bonding machine to interconnect IC conductive pads with appropriate substrate module conductive pads. Generally, the diameter or cross-sectional area of wire, the size and shape of the wire bonds, and the loop characteristics are selected as may be required to achieve a desired electrical performance, reliability, and bonding process throughput time.
While most IC to substrate module interconnections are made using wire bonding, wire bonding power ICs is a special case that requires larger diameter high purity wires to handle the higher power, higher current densities and high switching frequencies of power ICs.
It is known to wire bond power ICs such as field effect transistors, (FET), insulated gate bipolar transistors (IGBT), thyristors, and power diodes to substrate modules. Typical wire bonded power IC interconnection use wire comprising high purity aluminum with diameters ranging of 0.101-0.508 mm, (0.004 to 0.020 inches) to make individual interconnections. These wires have 10 or more times the cross-sectional area of wires used to bond other ICs to a substrate module.
Typically power ICs have only two or three interconnecting terminals such as a source terminal, a drain terminal and a gate terminal. Often power ICs are comprised of multiple individual devices that work together to perform switching or other functions. The individual devices are carefully sized and spaced to equally distribute dissipated power. These individual devices are interconnected using large area terminals that spread current and heat among individual devices that form the IC. These terminals also are used as the wire bonding terminals. Still many power ICs are mounted directly to a heat sink to dissipate thermal energy generated during operation. Power ICs are often used as high frequency switches forcing rapid on-off current flow. Undesirable parasitics, such as, parasitic inductance and parasitic resistance, associated with wire bond interconnection, can induce large voltage and current spikes in the IC leading to poor IC reliability, IC failure and even explosive failure of the IC. To compensate for interconnection parasitics, it has been necessary to over-design power IC circuits with increased design margins and this has lead to reduce power efficiencies and increases in the cost, volume and weight of the ICs.
Another problem caused by the rapid on-off current flow over conventional interconnecting wire bonds is that the bond wires are susceptible to rapidly varying electro-thermal-mechanical stress which can lead to interconnection failures. In particular, rapid changes in current flow produce rapid heating and therefore rapid thermal expansion and contraction of the larger diameter relatively stiff wire material. These changes continuously vary the loop length thereby producing cycling stresses on wire bond contact points at each end of the loop. This action can lead to fatigue failure at the wire bond contact points which will lead to increased current and therefore stress on remaining bond connections, thereby reducing the reliability of the overall electrical system.
Accordingly, there is a need in the art to increase the reliability and improve the performance of electrical interconnections between power ICs and substrate modules.